Well, yeah, it's pretty dry, so a fire is sketchy. Besides, you're day hiking, why would you need a fire?
But if you really think you need to, the general regulations basically say that you can't have one above 9600 ft and in certain areas, you have to use an existing fire ring (no making your own), it obviously has to be fully extinguished when you leave, and it needs to stay within a reasonable size and within that fire ring. That said, currently Yosemite is at what they call Stage 1 Fire Restrictions. With how dry and hot it is, wait much longer and it's probably going to Stage 2 pretty quickly. Then it's no fires anywhere in the park at all except at designated campgrounds and picnic areas.
So here's the official story:
http://www.nps.gov/yose/planyourvisit/f ... ctions.htm